The need for a new and more versatile personnel inspection system in mass transportation centers has increased in recent years. Extensive description of traditional and holographic methods is provided in U.S. Pat. No. 5,455,590, hereby incorporated by reference.
The holographic linear array of sequentially-switched millimeter-wave transmitter-receivers scanned quickly over a large aperture to actively illuminate the target as described in U.S. Pat. No. 5,455,590 uses a single frequency that is coherent, which means the phase of the returned signal is recorded as well as the amplitude. The holographic linear array described in U.S. patent application Ser. No. 08/440,279 improves image quality by using a wideband system.
U.S. Pat. No. 5,170,170 to Soumekh show a two-dimensional reconstruction process. In his book entitled FOURIER ARRAY IMAGING, Englwood Cliffs, N.J., Prentice Hall, 1994, Soumekh derived a reconstruction process for a circular aperture resulting in a two-dimensional image.
However, the previous systems lack the capability of scanning an individual from all sides quickly without inconveniencing the individual. Specifically, to fully inspect a person for concealed objects, it is necessary to image the person from many viewing angles. With a planar aperture, a person is required to pose at a number of predetermined positions while images are acquired. To obtain a sufficient number of images (8-128), total imaging time may be as high as several minutes, which is unacceptable for high throughput applications including airports.
Thus, there is a need for a three-dimensional holographic imaging method and apparatus that can provide high resolution with fast scanning and fast image reconstruction.